A derailleur—in particular a rear derailleur—normally comprises a fixed body (also called upper body, due to the position that it typically occupies with respect to the rest of the derailleur once mounted) and a mobile body (also called lower body), connected to the fixed body by means of a connection mechanism that allows a movement of the mobile body with respect to the fixed body; a chain guide is then mounted in an angularly moveable manner on the mobile body. The fixed body is mounted on the bicycle frame, near to the cogset, and the mobile body is moved with respect to it by acting on the connection mechanism, so as to move the chain guide in the axial direction of the cogset and thus obtain the desired changes in gear ratio. The chain guide is in constant engagement with the chain, both to move it in the axial direction guiding it in passing from one sprocket to another, and to keep it under tension so as to compensate for the changes in length of theoretical path of the chain due to different gear ratios; for this purpose, an elastic system (normally a spring) is thus arranged between the mobile body and the chain guide, so as to allow the latter to apply the desired tension on the chain.
It is furthermore known that the elastic system by itself is not always sufficient to ensure that the chain guide keeps the chain adequately under tension. Indeed, in certain cases the chain can oscillate excessively, particularly due to sudden stresses caused by uneven roads, by impacts or whatever other reason. Also due to the substantial mass of the chain, these oscillations can have the chain hits or slides against fixed parts of the bicycle; indeed, it is possible for the chain to drop (undesired disengagement from one of the toothed wheels of the transmission), with consequent blocking of the transmission of motion. These drawbacks are particularly troublesome in racing bicycles or in any case high-performance bicycles.
In order to avoid these drawbacks, derailleurs have been made in which between the mobile body and the chain guide there is not only the elastic system but also a damper, namely a mechanical-hydraulic component that exerts a braking action proportional to the relative speed of movement of the chain guide with respect to the mobile body: in the presence of normal movements due to gearshifting (which are relatively slow), the braking action is substantially zero, and therefore the damper does not obstruct gearshifting; in the presence of sudden stresses due to accidental causes, the damper intervenes with a braking action of increasing amount as the speed of movement increases. Typically, the damper comprises a first and a second variable volume chamber filled with a damping fluid and in connection with each other through a controlled fluid passage system; the angular movement of the chain guide determines changes in volume in the opposite direction of the first and of the second chamber and consequently a fluid overflow between the first and the second chamber through the controlled fluid passage system. The braking action is substantially proportional to the speed of passage of the fluid from one chamber to the other, therefore to the speed of angular movement of the chain guide.
Furthermore, in order to improve dynamic performance, unidirectional dampers have been proposed, i.e. capable of braking the movements in one direction but not in the opposite direction; thanks to the use of these dampers, it is possible to ensure that the chain guide is braked when the external stress tends to move it, but not when—the stress having ended—the elastic system tends to take it back into its correct position. The result is that the system tends not only to effectively oppose external stresses, but also to quickly return into its normal position once the stress has finished.
Derailleurs of this kind are known for example from US 2015/0072816 A1 and from US 2007/219029 A1.